Tubular needle,chiefly for writing with a jet of ink



S p 1969 E. ASCOLI 3,466,659

TUBULAR NEEDLE, CHIEFLY FOR WRITING WITH A JET OF INK Filed Sept. 8. 1966 INVENTOR firs 20 flscaL BY I I 4 TTORNEY United States Patent 13,478/ Int. Cl. G01d 15/16, 15/18 US. Cl. 346140 Claims ABSTRACT OF THE DISCLOSURE A needle for use in electrostatic inking apparatus includes a hollow cylinder whose outermost or front surface is wetted by the meniscus of the ink. This front surface is provided with a higher degree of roughness than the adjacent cylindrical surface from which it is separated by a sharp edge forming an angle of 90 between the two surfaces. The front surface is entirely wetted by the ink but the relatively smooth finish of the adjacent cylindrical surface prevents it from becoming wetted with the result that stability and uniformity of the projected ink image is attained.

The present invention has for its object a tubular needle, chiefly for incorporation with writing means resorting to jets of ink and a method for executing such a needle.

The Swiss Patent No. 356,477 relates to a writing method according to which a jet of ink is directed onto a surface by means of an electric field. According to such a method, ink is fed by a capillary tube and can be pulled off the end of said tube under the action of an electric field, so as to form a dash.

However, if special care is not exerted, the intensity of said dash may vary in an uncontrolled manner, as will appear from the following disclosure.

As illustrated in FIG. 1 of the accompanying drawings, the capillary tube 1 forms at its end a nozzle through which a jet of ink may be projected. The curves 2, 3 and 4 drawn in interrupted lines illustrate the shape which may be assumed at the output end of said nozzle under different operative conditions. In the absence of an electric field, the ink meniscus assumes the shape illustrated by the interrupted line 2 corresponding to an equilibrium between the surface pressure of the ink and the hydrostatic pressure prevailing inside the nozzle, in the case where the outer surface of the latter is not wetted by ink.

When the metal nozzle is brought to a suitable voltage with reference to the outer medium, or else, in the case of a non metallic nozzle associated with an electrically conductive ink, when the ink is brought to such a suitable voltage, the electric field produced gives birth to electrostatic forces which .act on the meniscus and deform the latter, which leads in its turn to a modification of the surrounding field. An oscillatory phenomenon arises then which consists in that the meniscus oscillates between the limit lines drawn at 3 and 4, each oscillation leading to the projection of a particle of ink 5 under the action of electrostatic forces. An input of ink fed from the inside of the nozzle replaces the amount projected at each oscillation.

The frequency f of the oscillatory phenomenon depends chiefly on the diameter d of the nozzle, which frequency decreases speedily when the value d increases, on the characteristic properties of the ink, such as surface pressure, viscosity, specific weight, dielectric constant and conductivity, and on the voltage.

Thus, for a given ink and a given voltage, the frequency of oscillation depends only on the diameter d "ice of the nozzle, the value of said diameter being that of the diameter of the actual area wetted by the meniscus, and stability of diameter governs the stability of the frequency. In the accompany drawings:

FIG. 1 is as already disclosed, an explanatory graph;

FIG. 2 is a further explanatory graph;

FIG. 3 is an elevational view of the improved arrangement according to the invention.

Turning to FIG. 2, it shows the end of a nozzle, the wall thickness of which is comparatively large with reference to the diameter of the nozzle. In such a case, and according to the extent of wetting of the terminal surface of the nozzle, the diameter of the meniscus may vary between d1 and d2 and it may also assume dissymmetrical shapes as shown at a and b.

The magnitude of the frequency can then vary in a reverse ratio with reference to the wetted surface and the ink projection may be considerably scattered.

The wetting conditions may vary, not only as a consequence of the geometrical shape given to the nozzle, but also for numerous other reasons such as the soiling of the ink and of the atmosphere, the state of cleanliness and the surface condition of the needle, the possibility of a chemical attack of the latter by the ink, etc.

It is therefore very important for the needle to show features such that the wetted surface may be as well defined and as stable as possible.

Furthermore, when the embodiment considered includes a plurality of needles projecting ink simultaneously or in succession, as described, for instance, in the US. Patent No. 3,136,594, it is essential for the needles to show practically identical properties as concerns the projection of ink so as to ensure identity of writing as to positioning, thickness of dash and frequency of projection, etc. for all the needles.

The object of the present invention consists in producing a needle satisfying said requirements, together with a method for allowing its production under conditions which are economically advantageous and allow also its accurate reproduction.

The term coeflicient of rugosity as used herein means the arithmetical mean of the differences of heights in the peaks and hollows of a roughened surface taken at an absolute value of a median line.

The improved needle according to the invention is characterized by the fact that its terminal section is constituted by a hollow cylinder the front section of which extending perpendicularly to its axis shows a coefficient of rugosity Ra of a magnitude of 0.5,u. while the adjacent cylindrical surface shows a coefficient of rugosity Ra of a magnitude of 0.05; the circular ridges separating said two areas being sharp and showing no burrs.

The improved method provided by the invention for the execution of such a needle consists in cutting the cylinder out of a tube of stainless steel the outer diameter of which is constant with a margin within 0.01 mm.

As illustrated diagrammatically and by way of example in the accompany FIG. 3, a preferred embodiment of said improved needle includes a fitting 6 of stainless steel turned on the lathe and inside which is inserted at 8 a hollow cylinder 7. The cylinder 7 is constituted by a section of a tube of stainless steel, the inner diameter of which is of a magnitude of 0.1 mm., while its outer diameter is of a magnitude of 0.2 mm. The front surface 9 of the cylinder 7 is carefully ground, said surface remaining however comparatively rough with reference to the condition of the peripheral surface 10 of the cylinder 7, which latter surface has to be trued so as to show a perfect finish. The circular ridge 11 separating the front surface from the peripheral surface of the cylinder must be sharp and show no burrs.

The standard value of the rugosity Ra of the terminal surface 9 should be of a magnitude of 0.5; and the value of the rugosity Ra of the peripheral surface should be of a magnitude of 0.5 Under such conditions, the circular front surface 9 is entirely wetted by the ink filling the capillary channel, while the finish of the peripheral surface of the cylinder prevents its wetting. The clearly defined ridge separating the two surfaces considered, the difference in the superficial condition of said surfaces and also the Well-defined angle of 90 between said surfaces ensure an accurate limitation of the wetted circle, account being taken of the surface pressure of the ink, of the material forming the tube and of the wetting angle obtained thereby. The diameter of the wetted surface is then identical with the outer diameter of the tube which is of a type actually available on the market and the length of which is the conventional length used for the production of hypodermic needles, with a margin for the outer diameter within 4. The identical operation of all the needles produced is thus obtained within a similar margin.

The needle may be produced as follows: I

The fitting 6 is obtained by turning from the bar and the cylinder 7 by cutting, after which the cylinder 7 is inserted into the fitting 6. Then the cylinder 7 is polished by a wheel, made of felt laden with an ultrafine diamond paste and a suitable lubricant, which wheel revolves at a high speed round an axis parallel with the axis of the needle revolving slowly in contact with the wheel. The technique is the same as that used for polishing metallographic test-pieces.

The transverse surface 9 is trued with a lapidarys mill of a suitable granular structure. The central opening which may be obstructed partly by the burrs produced by the preceding operation should be cleaned with a diamondcarrying steel wire, as well-known in the technique of the boring of artificial stones.

I claim:

1. A tubular needle, primarily for writing instruments operating by the projection of jets of ink, comprising a perforated support including a hollow terminal nozzle forming cylinder the front transverse surface of which shows a coefficient of rugosity of a magnitude of 0.5, and the peripheral surface of which adjacent said transverse surface shows a coefficient of rugosity of a magnitude of 0.05, said surface being clearly separated by a circular smooth well-defined ridge whereby the diameter of the" ink meniscus formed at the front transverse surface of said cylinder is identical with the outer diameter of said cylinder.

2. A needle as claimed in claim 1, wherein the cylinder has an outer diameter of a magnitude of 0.2 mm.

3.' A needle as claimed in claim 1, wherein the cylinder has an inner diameter of a magnitude of 0.1 mm.

4. A needle as claimed in claim 1, wherein the cylinder is made of stainless steel.

5. A needle as claimed in claim 1, wherein the cylinder is a stainless steel body the outer diameter of which is of a magnitude of 0.2 mm. and the inner diameter of which is of a magnitude of 0.1 mm.

References Cited UNITED STATES PATENTS 2,925,312 2/1960 Hollmann 346140X 3,060,429 10/1962 Winston 346- X JOSEPH W. HARTARY, Primary Examiner US. or. X.R. 346-75 

